defeasance

Definition from Nolo’s Plain-English Law Dictionary

The act of rendering something null and void, or a clause in a deed, lease, will, or other legal document that completely or partially negates the document if a certain condition occurs or fails to occur. For example, a will may provide that a gift of property is defeasable -- that is, void -- if the beneficiary fails to marry before a certain time.

Definition provided by Nolo’s Plain-English Law Dictionary.